Electronic circuit and method for automatically adjusting a phase of a drive signal applied to an electric motor in accordance with a zero current detected in a winding of the electric motor

ABSTRACT

A motor control circuit and associated techniques can adjust a phase of a motor drive to keep a rotational reference position of an electric motor at the same relative phase as a zero current in a motor winding at different motor speeds and as the motor accelerates and decelerates. In some embodiments, a particular circuit and technique can be used to detect the zero current in the motor winding.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

FIELD OF THE INVENTION

This invention relates generally to electric motor control circuits and,more particularly, to an electric motor control circuit that can providean automatic adjustment of a phase of drive signal applied to anelectric motor.

BACKGROUND OF THE INVENTION

Circuits to control and drive brushless DC (BLDC) electric motors areknown. In some arrangements, the circuits provide a phase advance ofdrive signals that drive the electric motor, the phase advance relatedto rotational speed of the electric motor or related to a measured motortotal current. However, such circuits are only able to provide onerelationship, or a small number of relationships, between phase advancesand rotational speeds. In addition, external components and pins of amotor control integrated circuit (IC) may be required to set theparameter for each electric motor or each electric motor application.

Some known electric motor drive circuits are described in U.S. Pat. No.7,590,334, issued Sep. 15, 2009, U.S. Pat. No. 7,747,146, issued Jun.29, 2010, and U.S. patent application Ser. No. 13/271,723, filed Oct.12, 2011, all of which are incorporated herein by reference in theirentireties and assigned to the assignee of the present invention.

A BLDC electric motor can exhibit different efficiency behaviors versusspeed when used in different applications. For example, the same BLDCelectric motor can be used with different fan blade arrangements indifferent applications. Different types of BLDC electric motors can alsoexhibit different efficiency behaviors versus speed.

Motor noise, vibration, and efficiency are influenced by a variety ofcharacteristics. One such characteristic is a phase of currents thatappear in the motor windings relative to a rotational position of themotor. Particularly as a motor speed increases or decreases, the phaseof the currents can lag or lead, respectively, a reference rotationalposition of the motor. Also, at high motor speeds, the current in themotor windings can tend to lag the reference position of the motor.

In view of the above, it would be desirable to provide an electric motorcontrol circuit and associated method that can generate electric motordrive signals having automatic phase adjustments determined inaccordance with a detected phase difference between motor windingcurrent and motor rotational position.

SUMMARY OF THE INVENTION

The present invention provides an electric motor control circuit andassociated method that can generate electric motor drive signals havingautomatic phase adjustments determined in accordance with a detectedphase difference between motor winding current and motor rotationalposition.

In accordance with one aspect of the present invention, a method ofdriving a multi-phase motor having a plurality of motor windingsincludes generating a zero current signal indicative of a zero crossingof a current though at least one of the plurality of motor windings;generating a position reference signal indicative of a referenceposition of an angular rotation of the motor; comparing a phase of thezero current signal with a phase of the position reference signal togenerate a phase comparison signal; generating a plurality of modulationsignals, each having a phase related to a value of the phase comparisonsignal; and generating a plurality of motor drive signals to theplurality of motor windings in accordance with the plurality ofmodulation signals.

In accordance with another aspect of the present invention, anelectronic circuit for driving a multi-phase motor having a plurality ofmotor windings includes a current measure module configured to generatea zero current signal indicative of a zero crossing of a current thoughat least one of the plurality of motor windings. The electronic circuitalso includes a position measure module configured to generate aposition reference signal indicative of a reference position of anangular rotation of the motor. The electronic circuit also includes amodulation signal generation module configured to compare a phase of thezero current signal with a phase of the position reference signal togenerate a phase comparison signal, and configured to generate aplurality of modulation signals, each having a phase related to a valueof the phase comparison signal. The electronic circuit also includes adrive circuit configured to generate a plurality of motor drive signalsto the plurality of motor windings in accordance with the plurality ofmodulation signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention, as well as the invention itselfmay be more fully understood from the following detailed description ofthe drawings, in which:

FIG. 1 is a block diagram of an exemplary motor control circuit having amodulation signal generation module and having a current measurementmodule;

FIG. 2 is a graph showing a variety of waveforms associated with theexemplary motor control circuit of FIG. 1, in particular, when the motorcontrol circuit is used to provide a sinusoidal drive to a motor;

FIG. 3 is another graph showing a variety of waveforms associated withexemplary motor control circuit of FIG. 1, in particular, when the motorcontrol circuit is used to provide a sinusoidal drive to the motor, andshowing phase differences between a current signal and a positionreference signal;

FIG. 4 is a block diagram of an exemplary modulation signal generationmodule that can be used as a modulation signal generation module of theexemplary motor control circuit of FIG. 1;

FIGS. 5 and 5A are block diagrams showing exemplary half bridge outputstages of the exemplary motor control circuit of FIG. 1, and showingdirections of motor winding current at different phases of operation;

FIG. 6 is a graph showing waveforms associated with the motor winding,in particular, a sinusoidal current, a modulation waveform associatedwith a sinusoidal drive of the motor winding, and a pulse widthmodulated (PWM) signal that drives a motor in accordance with themodulation waveform;

FIG. 7 is a pictorial showing details of positive and negative states ofthe PWM signal of FIG. 6;

FIG. 7A is a graph showing a PWM drive signal applied to an electricmotor, and showing a sinusoidal current associated with the PWM drivesignal;

FIG. 8 is a block diagram of another exemplary motor control circuithaving a modulation signal generation module and having a currentmeasure module in the form of a zero current detection module; and

FIG. 9 is a graph showing a variety of waveforms from which a zerocurrent in a motor winding can be detected, in particular, when atrapezoidal drive to a motor is used.

DETAILED DESCRIPTION OF THE INVENTION

Before describing the present invention, some introductory concepts andterminology are explained. As used herein, the term “modulationwaveform” is used to describe an envelope or characteristic function ofanother signal, for example, a pulse width modulated (PWM) signal.

Referring to FIG. 1, an exemplary motor control circuit 102 is coupledto drive an electric motor 104.

The motor 104 is shown to include three windings 104 a, 104 b, 104 c,which are each often depicted as a respective equivalent circuit havingan inductor in series with a resistor and in series with a back EMFvoltage source. For example, the winding A 104 a is shown to include aninductor 130 in series with a resistor 131 and in series with a back EMFvoltage source VA 136. The voltage of the back EMF voltage source VA 136is not directly observable when a current is flowing in an associatedmotor winding, but it directly observable when the current through theassociated winding is zero.

In general, the voltage across a motor winding, for example, across thewinding A 140 a, is governed by the following equation:VoutA−Vcommon=VA+IR+LdI/dt,

where:

VoutA=observable voltage at one end of the winding A;

Vcommon=voltage at junction of the windings 104 a, 104 b, 104 c;

R=resistance of the resistor 131;

L=inductance of inductor 130;

I=current through winding; and

VA=back EMF voltage

Thus, it can be seen that, if the current through the winding 104 a iszero, then VoutA=VA, which is an observable voltage.

The motor control circuit 102 includes a speed demand generator 107coupled to receive an external speed demand signal 106 from outside ofthe motor control circuit 102. The external speed demand signal 106 canbe in one of a variety of formats. In general the external speed demandsignal 106 is indicative of a speed of the motor 104 that is requestedfrom outside of the motor control circuit 102.

The speed demand generator 107 is configured to generate a speed demandsignal 107 a. A pulse width modulation (PWM) generator 108 is coupled toreceive the speed demand signal 107 a and configured to generate PWMsignals 108 a, a maximum duty cycle of which is controlled by the speeddemand signal 107 a. The PWM generator 108 is also coupled to receivemodulation waveforms 146 a, 146 b, 146 c from a modulation signalgeneration module 146. The PWM signals 108 a are generated with amodulation characteristic (i.e., a relative time-varying duty cycle) inaccordance with the modulation waveforms 146 a, 146 b, 146 c. Modulationwaveforms and associated PWM signals are described more fully below inconjunction with FIG. 6.

The motor control circuit 102 also includes a gate driver circuit 110coupled to receive the PWM signals 108 a and configured to generate PWMgate drive signals 110 a, 110 b, 110 c, 110 d, 110 e, 110 f to drive sixtransistors 112, 114, 116, 118, 120, 122 arranged as three half bridgecircuits 112/114, 116/118, 120/122. The six transistors 112, 114, 116,118, 120, 122 operate in saturation to provide three motor drive signalsVoutA, VoutB, VoutC, 124, 126, 128, respectively, at nodes 102 d, 102 c,102 b, respectively.

The motor control circuit 102 can also include a position measurementmodule 142, which can be coupled to receive either back EMF signal(s)(e.g., can be coupled to receive one or more of the motor drive signals124, 126, 128, which include back EMF signals directly observable attimes when the motor windings 104 a, 104 b, 104 c are not being drivenand respective winding currents are zero) or Hall element signals fromHall elements (not shown). The position measure module 142 is configuredto generate a position reference signal 142 a indicative of a rotationalreference position of the motor 104.

The motor control circuit 102 can also include a current measurementmodule 144, which can be coupled to receive one of the motor drivesignals 124, 126, 128. The current measurement module 144 is configuredto generate a zero current signal 144 a indicative of a zero crossing ofthe current through one or more of the motor windings. An exemplarycurrent measurement module is described in further detail below inconjunction with FIG. 8.

The modulation signal generation module 146 is coupled to receive theposition reference signal 142 a and the zero current signal 144 a. Themodulation signal generation module 146 is configured to change a phaseof the modulation waveforms 146 a, 146 b, 146 c in accordance with aphase difference between the position reference signal 142 a and thezero current signal 144 a. An exemplary modulation signal generationmodule 146 is described below in conjunction with FIG. 4.

The motor control circuit 102 can be coupled to receive a motor voltageVMOT, or simply VM, at a node 102 a, which is supplied to the motorthrough the transistors 112, 116, 120 during times when the uppertransistors 112, 116, 120 are turned on. It will be understood thatthere can be a small voltage drop (for example, 0.1 volts) through thetransistors 112, 116, 120 when they are turned on and supplying currentto the motor 104.

As described above, the motor control circuit 102 can automaticallyadjust a timing, i.e., a phase, of the drive signals 124, 126, 128 inrelation to a sensed rotational position of the motor 104.

Referring now to FIG. 2, graphs 200, 220, 240, and 260 have a horizontalaxes with scales in units of time in arbitrary units. The graphs 200,220, 240 have vertical axes with scales in units of voltage in arbitraryunits. The graph 260 has a vertical axis with a scale in units ofcurrent in arbitrary units.

A signal 202 is representative of a back EMF signal (i.e., a voltagesignal) on one of the motor windings (e.g., winding A 104 a) of themotor 104 of FIG. 1 when the motor 104 is spinning. The back EMF voltage202 is generally sinusoidal.

In some embodiments of the motor control circuit 102 of FIG. 1, the zerocrossing of the back EMF signal 202 can be used by the positionmeasurement module 142 to identify a reference rotational position ofthe motor 104. It is desirable that the zero crossing of the back EMFsignal 202 at the time 208 be coincident or nearly coincident with azero current passing through the motor winding upon which the back EMFsignal 202 is generated. Such a relationship will result in mostefficient motor operation.

In some embodiments of motor control circuit 102, a back EMF signal isnot used to detect rotational position of the motor 104. Instead, Hallelements are positioned about the motor 104 and Hall element signals222, 224, 226 are generated as the motor 104 rotates. It should beapparent that the signals 222, 224, 226 are representative of rotationalpositions of the motor 104. Characteristically, it can be seen that noneof the transitions of the Hall element signals 222, 224, 226 align withthe zero crossing of the back EMF signal 202. Nevertheless, the time 208can be identified by the signals 222, 224, 226 as being part way, e.g.,halfway, between particular transitions of the signals 222, 224, 226.

Signals 242, 244, 246 are representative of the above-describedmodulation waveforms 146 a, 146 b, 146 c of FIG. 1. The modulationwaveforms 242, 244, 246 are used to generate PWM signals to drive themotor 104. Correspondence between the modulation waveforms 242, 244, 246and the PWM signals is described more fully below in conjunction withFIG. 6.

It will be recognized that the modulation waveform 242 is associatedwith the winding A 104 a of FIG. 1 and generally aligns with the backEMF signal 202 that is associated with the same winding. The othermodulation waveforms 244, 246 are associated with the other windings B,C 104 b, 104 c, respectively, of the motor 104 of FIG. 1.

Signals 262, 264, 266 are representative of currents that appear on thewindings A, B, C 104 a, 104 b, 104 c, respectively, of the motor 104 ofFIG. 1. It will be understood that the actual current signals on themotor windings may be more complex than those shown in the signals 262,264, 266. However the current signals 262, 264, 266 are representativeof an average current versus time through the three motor windings. Itwill be understood that the current 262 on the motor winding A 104 a isgenerally in phase with the back EMF signal 202. However, there can be aphase difference between the current signal 262 and the associated backEMF signal 202 as described more fully below in conjunction with FIG. 3.

An electrical revolution of the motor 104 can be divided into sixstates, or time periods, 201 a, 201 b, 201 c, 201 d, 201 e, 2011

Referring now to FIG. 3, a graph 300 has a horizontal axis with a scalein units of time in arbitrary units. The graph 300 also has a verticalaxis with a scale in units of voltage and current in arbitrary units. Agraph 320 has a horizontal axis with a scale in units of time inarbitrary units. The graph 320 also includes a vertical axis with ascale in units of voltage in arbitrary units.

A signal 304 is representative of a current signal on the winding A 104a of FIG. 1. Thus, the signal 304 corresponds to the signal 262 of FIG.2. A signal 302 is representative of a back EMF signal 136 on thewinding A104 a of FIG. 1. Thus, the signal 302 corresponds to the signal202 of FIG. 2. Zero crossings of the signals 302, 304 will be apparent.A time difference 308 is indicative of a time difference between a zerocrossing of the back EMF signal 302 and a zero crossing of the currentsignal 304. Therefore, the time difference 308 is representative of atime difference between a rotational position reference (i.e., the zerocrossing of the back EMF signal 302) and a zero current passing throughthe associated motor winding.

A signal 306 is representative of a current signal on the winding A 104a of FIG. 1 during a time period during which the motor 104 isaccelerating in rotational speed or during which the motor 104 isrotating at high speed. It can be seen that relative phases haveshifted. The zero crossing of the current signal 306 is retarded withrespect to the zero crossing of the back EMF signal 302. The zerocrossing of the back EMF signal 302 is indicative of a referencerotational position of the motor 104. The zero crossing of the currentsignal 306 is representative of a zero current through the motor windingA 104 a. It is desirable that the zero crossings coincide in time andphase. A lack of time coincidence will result in increased motor noiseand vibration, and decreased motor efficiency.

A modulation waveform 322 is the same as or similar to the modulationwaveform 242 of FIG. 2. Thus, when the motor is accelerating or rotatingrapidly, it can be seen that the current signal 306 is retarded relativeto the modulation waveform 322. It would be desirable to advance themodulation waveform 322 (i.e., move the modulation waveform 322 to theleft) to advance the current signal 306 so that a zero crossings of thecurrent signal 306 can occur coincident or nearly coincident with thezero crossing of the back EMF signal 302, which is indicative of arotational reference position of the motor 104.

In general, the modulation signal generation module 146 of FIG. 1 isable to advance or retard the various modulation waveforms 146 a, 146 b,146 c in accordance with a received rotational position reference signal142 a and in accordance with the zero current signal 144 a, of which theback EMF signal 302 and the current signals 304, 306 are representative.

For conventional sinusoidal motor drive signals, such as those describedabove in conjunction with FIGS. 2 and 3, for reasons described above inconjunction with FIG. 1, it is not readily possible to observe anddetect a zero crossing of the back EMF signal 302, because each one ofthe motor windings 104 a, 104 b, 104 c is constantly being driven. Inorder to observe a back EMF signal, it is necessary to at leastmomentarily stop a drive signal to a motor winding. Thus, with asinusoidal motor drive signal arrangement, in some embodiments, thesinusoidal drive signal to at least one of the windings 104 a, 104 b,104 c of the motor 104 can be stopped for a small time window in orderto observe the zero crossing of the back EMF signal. To this end, in themotor control circuit 102 of FIG. 1, a control signal 142 b can beprovided by the position measurement module 142 to the gate driver 110.

Referring now to FIG. 4, a modulation signal generation module 402 canbe used as the modulation signal generation module 146 of FIG. 1.

The modulation signal generation module 402 is coupled to receive adetected position reference signal 414 and a detective zero currentsignal 418. The detected position reference signal 414 can be the sameas or similar to the position reference signal 142 a of FIG. 1. Thedetected zero current signal 418 can be the same as or similar to thezero current signal 144 a of FIG. 1.

As described above, the detected position reference signal 414 can begenerated in conjunction with zero crossings of a back EMF signal usinga sinusoidal drive waveform by using a short time period during whichthe sinusoidal drive to a winding is stopped. In other embodiments, thedetected position reference signal 414 can be generated in conj unctionwith Hall elements disposed around the motor 104 of FIG. A andassociated Hall element signals.

The modulation signal generation module 402 is also coupled to receive asystem clock signal 416 having a fixed high frequency.

A so-called “theta ramp generator” 404 is coupled to receive thedetected position reference signal 414 and the system clock signal 416.The theta ramp generator 404 is configured to generate an unadjustedtheta signal 404 a, which can be a digital, signal comprised of asequence of values representative of a ramp signal that periodicallyreaches a terminal value and resets to zero. The reset time of the rampsignal is fixed in relation to the position reference (i.e., a fixedrotational position of the motor 104) of which the detected positionreference signal 414 is indicative.

In operation, the theta ramp generator 404 can identify a time period,measured by counting a number of system clock transitions, betweenposition references identified by the detected position reference signal414. In other words, the theta ramp generator 404 can identify a time(i.e., a number of transitions of the system clock signal 416) that ittakes the motor 104 to turn through one electrical rotation. The thetaramp generator 404 can divide the identified number of transitions ofthe system clock 416 by a fixed scalar, for example, by 256. Thus, amotor electrical revolution can be divided into 256 parts. Accordingly,the clock signal 402 can have a frequency that achieves, for example,256 transitions during one electrical revolution of the motor. The clocksignal 402 can be generated by and used by the theta ramp generator 404to generate a rate at which ramp values of the unadjusted theta signal404 a are incremented and output within the unadjusted theta signal 404a. Thus, it will be understood that resets to zero of the ramp signal inthe unadjusted theta signal 404 a are achieved once for every electricalrevolution of the motor, and there can be, for example, 256 steps in theramp.

A timing/phase error detector 410 is coupled to receive the detectedzero current signal 418, coupled to receive the detected positionreference signal 414, and coupled to receive the clock signal 402.

The timing/phase error detector 410 is configured to identify a timedifference (i.e., a phase difference) between a position referenceidentified by the detected position reference signal 414 and a zerocurrent crossing identified by the detected zero current, signal 418.

Referring briefly again to FIG. 3, in other words, the timing/phaseerror detector 410 is operable to identify a time difference between thezero crossing of the current signal 304 or 306 and a zero crossing of aback EMF voltage signal 302.

Referring again to FIG. 4, the timing/phase error detector 410 isconfigured to generate an error signal 410 a, which, in someembodiments, can be a digital value, representative of the identifiedtime (i.e., phase) difference.

A proportional integrator differentiator (PID), or, in otherembodiments, a proportion integrator (PI), can be coupled to receive theerror signal 410 a, and configured to essentially filter the errorsignal 410 a to generate an adjustment signal 412 a. In someembodiments, the adjustment signal 412 a can be a digital valueproportional to the time difference identified by the timing/phase errordetector 410.

A summing module 406 is coupled to receive the unadjusted theta signal404 a (i.e., a sequential set of digital values representative of aresetting ramp signal at a fixed phase), coupled to receive theadjustment signal 412 a, and configured to generate a theta signal 406a.

In operation, it should be understood that the theta signal 406 a is aresetting ramp signal like the unadjusted theta signal 404 a, but forwhich the reset times of the ramp signal are moved in time, i.e.,adjusted phase, in accordance with a value of the adjustment signal 412a.

A modulation profile look up table and processor 408 is coupled toreceive the theta signal 406 a. The modulation profile look up table andprocessor 408 is configured to store therein value representative of oneor more modulation profiles, for example, the modulation profile 242 ofFIG. 2.

In operation, the theta signal 406 a is used to sequence between valuesof the stored modulation signal within the modulation profile look uptable and processor 408. It will be understood that, a phase of thetheta signal 406 a, of which reset portions of the theta signal 406 aare representative, is adjustable according to a time difference betweena position reference identified within the detected position referencesignal 414 and a zero crossing of the current in a motor winding asidentified within the detected zero crossing signal 418. Accordingly, aphase, i.e., a timing of, a modulation signal 408 a generated by themodulation profile look up table and processor 408 is adjustable.

A processor portion of the modulation profile look up table andprocessor 408 can automatically generate other modulation profiles 408b, 408 c at other fixed, phases, for example, the modulation profiles244, 246 of FIG. 2, which can be at fixed phases relative to themodulation profile 408 a, e.g., the modulation profile 242 of FIG. 2.

Exemplary circuits and methods to detect a zero current through motorwindings are described below in conjunction with FIGS. 5-8. However, itshould be appreciated that other methods can be used to detect a zerocurrent passing through a motor winding.

Referring now to FIG. 5, three half bridge circuits 502, 504, 506correspond to the three half bridge circuits 112/114, 116/118, 120/122of FIG. 1 and are shown driving three motor windings. Currents throughthe half bridge circuit 502 and through one of the motor windings areindicated by dashed lines identified by circled numbers 1, 2, and 3. Thecurrents 1, 2, and 3 are indicative of currents at different timesthrough the half bridge circuit 502 during a positive polarity of acurrent signal in the motor winding, e.g., during positive parts of thecurrent signal 262 of FIG. 2. The current 1 is indicative of the upperFET being on, the current 3 is indicative of the lower FET being on, andthe current 2 is indicative of both FETS being turned off. It will beappreciated that the current 2 passes through an intrinsic diode of thelower FET, and thus, the voltage VoutA (see e.g., signal 124 of FIG. 1)achieves a voltage approximately 0.7 volts below ground beginning whenthe two FETs of the half bridge 502 are turned off and returns to groundvoltage when the lower FET turns on. Thus, it will be appreciated that,by detecting the voltage VoutA going below ground and returning toground, an actual zero current through the half bridge 502 and throughthe associated motor winding can be identified.

Referring now to FIG. 5A, in which like elements of FIG. 5 are shownhaving like reference designations, currents through the half bridgecircuit 502 and through one of the motor windings are again indicated bydashed lines identified by circled numbers 1, 2, and 3. The currents 1,2, and 3 are indicative of currents at different times through the halfbridge circuit 502 during a negative polarity of a current signal in themotor winding, e.g., during negative parts of the current signal 262 ofFIG. 2. The current 1 is indicative of the upper FET being on, thecurrent 3 is indicative of the lower FET being on, and the current 2 isindicative of both FETS being turned off. It will be appreciated thatthe current 2 passes through an intrinsic diode of the upper FET, andthus, the voltage VoutA achieves a voltage approximately 0.7 volts abovethe voltage VM beginning when the two PETs of the half bridge 502 areturned off and returns to the voltage VM when the upper FET turns onThus, it will be appreciated that, by detecting the voltage VoutA goingabove the voltage VM and returning to the voltage VM, an actual zerocurrent through the half bridge 502 and through the associated motorwinding can be identified.

Referring now to FIG. 6, a graph 600 has a horizontal axis with a scalein units of time in arbitrary units and a vertical axis with a scale inunit of current in arbitrary units. A graph 620 has a horizontal axiswith a scale in units of time in arbitrary units and a vertical axiswith a scale in unit of voltage in arbitrary units. A graph 640 has ahorizontal axis with a scale in units of time in arbitrary units and avertical axis with a scale in unit of voltage in arbitrary units.

A signal 602 is representative of a current signal in a motor winding Awhen a sinusoidal drive signal is used. The current signal 602 can bethe same as or similar to the current signal 262 of FIG. 2. As describedabove, when a PWM drive signal is used, the current signal 602 canappear more complex, but the signal 602 is generally representative ofan average current through the winding A. The current signal has zerocrossings at time 606, 608.

A modulation signal 622 can be the same as or similar to the modulationsignal 242 of FIG. 2. The modulation signal 622 can have six timeperiods or phases, of which four are shown 604 a, 604 b, 604 c, 604 d.

A PWM signal 642 can be generated in accordance with the modulationwaveform 622 and can have times of high duty cycle 642 a, 642 b at timesof peaks 622 a, 622 b of the modulation waveform 622 and times of lowerduty cycle at times of other portions of the modulation waveform 622 inaccordance with values of the modulation waveform 622. The PWM signal642 can be the signal actually applied to the motor winding A 104 a ofthe motor 104 of FIG. 1 for a sinusoidal drive.

Referring now to FIG. 7, PWM pulses 702, 702′ are indicative of the PWMpulses 642 of FIG. 6 during negative polarity portions of the currentsignal 602. A PWM pulse 704 is indicative of the PWM pulses 642 of FIG.6 during positive polarity portions of the current signal 602.

The PWM pulses 702, 702′ have raised or transient portions 702 b, 702 c,702 b′, 702 c′ and steady state portions 702 a, 702 a′. In accordancewith the discussion above in conjunction with FIGS. 5 and 5A, it will beappreciated that, when both transistors, e.g., FETs, of a half bridgeare turned off, a voltage VoutA appearing on the associated motorwinding temporarily goes above the motor voltage VM, or below ground,depending upon a polarity of the current in the motor winding, i.e., apolarity of the current signal 602. It will also be appreciated thateach major edge transition of the PWM signals 702, 704, 702′ is precededby a short time period during which both of the FETs are turned off,otherwise, both FETs might simultaneously turn on, resulting in a shortcircuit between the motor voltage VM and ground. Thus, when the two FETsare turned off, the transient signal portions 702 b, 702 c, 704 b, 704c, 702 b′, 702 c′ result. The transient signal portions 702 b, 702 c,704 b, 704 c, 702 b′, 702 c′ can occur for a short time period, forexample, for about five hundred nanoseconds.

It will be apparent that the direction of the transient voltage signalportions 702 b, 702 c, 704 b, 704 c, 702 b′, 702 c′ change directionupon each occurrence of a zero crossing of the current signal 602, i.e.,at times 606, 608. Thus, detection of changes in direction of thetransient signal portions 702 b, 702 c, 704 b, 704 c, 702 b′, 702 c′ canbe used to identify a zero current in the associated motor winding.

Referring now to FIG. 7A, a graph 720 has a horizontal axis with a scalein units of time in arbitrary units and a vertical axis with a scale inunit of voltage in arbitrary units. A graph 740 has a horizontal axiswith a scale in units of time in arbitrary units and a vertical axiswith a scale in units of current in arbitrary units.

A signal 722 is representative of the PWM signal 642 of FIG. 6, butshowing transient signal portions like the transient portions 702 b, 702c, 704 c, 704 d, 702 b′, 702 c′ of FIG. 7. A signal 742 is the same asor similar to the current signal 602 of FIG. 6.

Times t1-t9 occur during the transient signal portions. From discussionabove in conjunction with FIGS. 5 and 5A, it will be apparent that thetransient signal portions occur when both FETs of an associated halfbridge circuit driving a motor winding are turned off.

For reasons described above in conjunction with FIGS. 5, 5A, and 7, atthe time t6, the transient signal portion changes orientation coincidentor nearly coincident with a zero crossing of the current signal 742.Thus, the change of orientation of the transient signal portions can beused to detect a zero current crossing in a motor winding. Inparticular, at the times t1-t5, the transient signal portions extendabove the motor voltage VM. Conversely, at the times t6-t9, thetransient signal portions extend below ground. Another change ororientation (not shown) of the transient signal portions occurs at anext zero crossing of the current signal 742, and can also be used todetect the next zero crossing.

In some embodiments, a circuit (see, e.g., comparators 808, 810 shownbelow in FIG. 8) can be operable to sample the signal VoutA 722 todetect the signal transient portions only at or near to the times t1-t9,and also at other similar times that follow. The times t1-t9 and similartimes that follow are known, since both FETs are momentarily turned offat those times. In other embodiments, the signal 722 can be continuallysampled to detect the transient signal portions.

In some embodiments, the changes of orientations of the transient signalportions can be detected with two comparators. Both zero crossings ofthe current signal 742 can be detected. However, in other embodiments,one comparator can be used to detect the presence or absence oftransient signal portions that either extend upward or that extenddownward. Still, both zero crossings of the current signal 742 can bedetected with one comparator.

Referring now to FIG. 8, in which like elements of FIG. 1 are shownhaving like reference designations, a zero current detection module 802can be the same as or similar to the current measurement module 144 ofFIG. 1.

The zero current detection module 802 can include a first comparator 808coupled to the three motor windings via a selectable switch 804. Thezero current detection module 802 can also include a second comparator810 coupled to the three motor windings via a selectable switch 806. Thefirst comparator 808 can be coupled to receive a reference voltage equalto or close to the motor voltage VM. The second comparator 810 can becoupled to receive a reference voltage equal to or close to ground.

The first comparator 808 is configured to generate an output signal 808a indicative of voltage on a selected motor winding going above themotor voltage. The second comparator 810 is configured to generate anoutput signal 810 a indicative of voltage on a selected motor windinggoing below ground. Thus, in operation, the first comparator 808operable to detect the positive transient signal portions 702 b, 702 c,702 b′, 702 c′ of the PWM signal of FIG. 7 associated with a sinusoidalmotor drive. Similarly, in operation, the second comparator 810 isoperable to detect the negative transient signal portions 704 b, 704 cof the PWM signal of FIG. 7 associated with a sinusoidal motor drive. Asdescribed above, edges of these signal portions can be used to identifya zero current crossing in an associated motor winding.

The zero current detection module 802 can also include a multiplexer 812coupled to receive the output signal 808 a, 810 a and configured togenerate and output single 812 a representative of a selected one of theoutput signals 808 a, 810 a.

The multiplexer 812 can be coupled to receive a control signal 146 dfrom the modulation signal generation module 146. The switches 804, 806can be coupled to receive other control signals (not shown) from themodulation signal generation module 146.

The modulation signal generation module 146 can use a variety of typesof logic to identify a zero current crossing in one or more of the motorwindings. For example, in accordance with the discussion above inconjunction with FIGS. 7 and 7A, for a PWM sinusoidal motor drivesignal, the output signals 808 a, 810 a can be used to identify a changein direction of the transient signal portions 702 b, 702 c, 704 b, 704c, 702 b′, 702 c′ of the PWM signal 642. Essentially, the multiplexer812 can switch to view the other comparator whenever detection is madeof a particular direction of the transient signal portions.

In some embodiments, the switches are not used and only one motorwinding is used to provide signals to the comparators 808, 810. In someembodiments, only one comparator is used and the multiplexer 812 is notnecessary. A variety of different types of logic can be used by themodulation signal generation module 146 to identify a zero crossing of acurrent through a motor winding by using the above described techniqueof detecting when a voltage on a motor winding goes above the motorvoltage VM and/or below ground.

Referring now to FIG. 9, a graph 900 has a horizontal axis with a scalein units of time in arbitrary units. The graph 900 also has a verticalaxis with a scale in units of current in arbitrary units. A graph 920has a horizontal axis with a scale in units of time in arbitrary units.The graph 920 also has a vertical axis with a scale in units of voltagein arbitrary units.

A signal 904 is representative of a trapezoidal motor drive, as opposedto the sinusoidal motor drive signals described above. The signal 904 isrepresentative of a trapezoidal current signal on a motor winding.

It will be understood that the voltage waveform 922 is representative ofan actual voltage applied to a motor winding for a one hundred percentmotor drive. For one hundred percent motor drive, the signal 922achieves a voltage of VM (motor voltage) with a duty cycle of onehundred percent, and at other times is zero. For a different trapezoidalmotor drive of less than one hundred percent (not shown), during thetime period that the one hundred percent drive signal 922 achieves thevoltage of VM, the different trapezoidal motor drive provides a pulsewidth modulated signal that has a pulse width modulation with a dutycycle in accordance with the motor drive of less than one hundredpercent.

The time of a motor electrical revolution can be broken into six states,with only four of the states 902 a, 902 b, 902 c, 902 d shown. Signalsduring the other two states will be apparent. Each one of the motorwindings receives a motor drive signal like the motor drive signal 922,but shifted in phase and beginning at a different one of the phases.

With a trapezoidal drive, a drive signal applied to a motor winding iszero during the first phase 902 a and also zero during the fourth phase902 d. Thus, during the first and fourth phases 902 a, 902 d, thecurrent signal 904 achieves a zero current during a signal portion 904 aand during a signal portion 904 d. The zero current is not achievedimmediately at the beginning of the first and fourth phases 902 a, 902 ddue to inductive behavior of the motor winding. For reasons describedabove in conjunction with FIG. 1, when the drive voltage applied to awinding is zero, and the current decays to zero, the back EMF voltage isdirectly observable across the winding.

For reasons described more fully above in conjunction with FIGS. 5 and5A, during a signal portion 922 a, the signal 922 on the motor windingachieves a voltage of VM+Vd, and, during a signal portion 922 d, thesignal 922 achieves a voltage of −Vd. An exemplary method of detectingthe zero current during signal portions 904 a, 904 d is described abovein conjunction with FIGS. 5 and 5A. To this end, parts 922 a, 922 d ofthe signal 922 can be used to detect the zero winding current usingcircuits and techniques such as those described above in conjunctionwith FIGS. 5, 5A and 8.

During a portion of the first and fourth phases 902 a, 904 d, inparticular, during dashed parts 922 b, 922 e of the voltage signal 922,the back EMF voltage is directly observable. During the parts 922 b, 922e of the voltage signal 922, and also during parts 922 a, 922 d, nodrive signal is applied to an associated motor. As described above,inductive behavior of the motor winding causes the current through themotor winding to achieve zero current only during the parts 922 b, 922 eof the drive signal 922.

From the above discussion, it should be apparent that, with atrapezoidal drive and using six motor drive states, there aresubstantial time periods when each motor winding is not being driven,for example, the motor winding is not being driving during time periodsassociated with signal portions 922 a, 922 b that together occupy onesixth (i.e., sixty degrees) of a motor electrical rotation and duringtime periods associated with signal portions 922 d, 922 e that togetheroccupy another one sixth (i.e., sixty degrees) of a motor electricalrotation. The current through the motor winding becomes zero at the endsof transient signal portions 922 a, 922 d, i.e., during signal portions922 b, 922 e, During the signal portions 922 b, 922 e, the back EMFvoltage is directly observable. Thus, unlike the sinusoidal drivedescribed above, for which motor windings are continuously driven, withthe six state trapezoidal drive arrangement, there is no need toseparately generate a time window during which no motor drive is appliedto the windings in order to detect the transient signal portions 922 a,922 d, ends of which are indicative of zero winding current, or todetect a zero crossings during the signal portions 922 b, 922 e, whichare indicative of motor rotational position.

All references cited herein are hereby incorporated herein by referencein their entirety.

Having described preferred embodiments, which serve to illustratevarious concepts, structures and techniques, which are the subject ofthis patent, it will now become apparent to those of ordinary skill inthe art that other embodiments incorporating these concepts, structuresand techniques may be used. Accordingly, it is submitted that that scopeof the patent should not be limited to the described embodiments butrather should be limited only by the spirit and scope of the followingclaims.

What is claimed is:
 1. A method of driving a multi-phase motor having aplurality of motor windings, the method comprising: generating a zerocurrent signal indicative of a zero crossing of a current though atleast one of the plurality of motor windings; generating a positionreference signal indicative of a reference position of an angularrotation of the motor; comparing a phase of the zero current signal witha phase of the position reference signal to generate a phase comparisonsignal; generating a plurality of modulation signals, each having aphase related to a value of the phase comparison signal; and generatinga plurality of motor drive signals to the plurality of motor windings inaccordance with the plurality of modulation signals, wherein thegenerating the plurality of modulation signals comprises: providing alook up table in which modulation values are stored corresponding to ashape of at least one of the plurality of modulation signals; generatinga first continuous sawtooth ramp signal having a smallest value and alargest value and a plurality of values between the smallest and largestvalues; adding to the first continuous sawtooth ramp signal a valuerelated to the phase comparison signal to generate a second continuoussawtooth ramp signal having the smallest value and the largest value andthe plurality of values between the smallest and largest values, whereinthe smallest and largest values of the second continuous sawtooth rampsignal are shifted in time by an adjustment time from the smallest andlargest values of the first continuous sawtooth ramp signal, wherein theadjustment time is related to the phase comparison signal; using theadjusted continuous sawtooth ramp signal to sequentially look up valuesin the look up table to generate the at least one of the plurality ofmodulation signals; and generating at least one other modulation signalat a predetermined phase relationship to the at least one of theplurality of modulation signals.
 2. The method of claim 1, wherein thegenerating the position reference signals comprises: generating a Hallsensor signal with a Hall sensor disposed proximate to the motor.
 3. Themethod of claim 1, wherein the generating the position reference signalscomprises: generating a back EMF signal with a back EMF module coupledto at least one of the plurality of motor windings.
 4. The method ofclaim 1, wherein the generating the position reference signal comprises:stopping a motor drive signal to at least one of the plurality of motorwindings during a time window proximate to the motor achieving thereference position; and detecting the reference position during the timewindow by a zero crossing of a back EMF signal.
 5. The method of claim1, wherein the generating the zero current signal comprises: generatingthe plurality of motor drive signals with a respective plurality of halfbridge circuits coupled to the electric motor, each half bridge circuitcomprising: respective first and second series coupled transistors; arespective power supply high voltage node for receiving a high powersupply voltage; a respective power supply low voltage node for receivinga low power supply voltage; and a respective output node at which arespective one of the plurality of motor drive signals is generated;detecting a reverse current passing through at least one of the first orthe second transistors of at least one of the plurality of half bridgecircuits, wherein the detecting comprises at least one of: detecting avoltage at the output node that is above the high power supply voltage;or detecting a voltage at the output node that is below the low powersupply voltage; and generating a zero current signal indicative of azero crossing of a current though at least one of the plurality of motorwindings in accordance with the detecting the reverse current.
 6. Themethod of claim 5, wherein the detecting the reverse current comprises:detecting the reverse current by sampling a voltage at the output nodeonly at times when the first and second transistors are both turned off.7. The method of claim 1, wherein each one of the plurality of motordrive signal comprises: a respective plurality of pulse width modulatedsignals, each one of the plurality of pulse width signals having highstates comprising steady state high values proximate to the high powersupply voltage and transient high values higher than the higher powersupply voltage, and each one of the plurality of pulse width signalsalso having low states comprising steady state low values proximate tothe low power supply voltage and transient low values lower than thelower power supply voltage.
 8. An electronic circuit for driving amulti-phase motor having a plurality of motor windings, the electroniccircuit comprising: a current measure module configured to generate azero current signal indicative of a zero crossing of a current though atleast one of the plurality of motor windings; a position measure moduleconfigured to generate a position reference signal indicative of areference position of an angular rotation of the motor; a modulationsignal generation module configured to compare a phase of the zerocurrent signal with a phase of the position reference signal to generatea phase comparison signal, and configured to generate a plurality ofmodulation signals, each having a phase related to a value of the phasecomparison signal; and a drive circuit configured to generate aplurality of motor drive signals to the plurality of motor windings inaccordance with the plurality of modulation signals, wherein themodulation signal generation module comprises: a look up table in whichmodulation values are stored corresponding to a shape of at least one ofthe plurality of modulation signals; a sawtooth generator configured togenerate a first continuous sawtooth ramp signal having a smallest valueand a largest value and a plurality of values between the smallest andlargest values; a timing/phase error detector coupled to receive asignal representative of the zero current signal, coupled to receive asignal representative of the position reference signal, and configuredto generate a signal representative of the phase comparison signal; anda summation module configured to add to the first continuous sawtoothramp signal a value related to the phase comparison signal to generate asecond continuous sawtooth ramp signal having the smallest value and thelargest value and the plurality of values between the smallest andlargest values, wherein the smallest and largest values of the secondcontinuous sawtooth ramp signal are shifted in time by an adjustmenttime from the smallest and largest values of the first continuoussawtooth ramp signal, wherein the adjustment time is related to thephase comparison signal, wherein the adjusted continuous sawtooth rampsignal is used to sequentially look up values in the look up table togenerate the at least one of the plurality of modulation signals andalso used to generate at least one other modulation signal at apredetermined phase relationship to the at least one of the plurality ofmodulation signals.
 9. The electronic circuit of claim 8, wherein theposition measure module is further configured to generate the positionsignal in accordance with a Hall signal generated by a Hall sensordisposed proximate to the motor.
 10. The electronic circuit of claim 8,wherein the position measure module is further configured to generatethe position signal in accordance with a back EMF signal generated in amotor winding.
 11. The electronic circuit of claim 8, wherein thegenerating the position reference signal comprises: stopping a motordrive signal to at least one of the plurality of motor windings during atime window proximate to the motor achieving the reference position; anddetecting the reference position during the time window by a zerocrossing of a back EMF signal.
 12. The electronic circuit of claim 8,further comprising: a plurality of half bridge circuits coupled to theelectric motor and configured to generate the plurality of motor drivesignals, each half bridge circuit comprising: respective first andsecond series coupled transistors; a respective power supply highvoltage node for receiving a high power supply voltage; a respectivepower supply low voltage node for receiving a low power supply voltage;and a respective output node at which a respective one of the pluralityof motor drive signals is generated; at least one comparator configuredto generate a respective at least one comparator output signalindicative of a reverse current passing through at least one of thefirst or the second transistors of at least one of the plurality of halfbridge circuits by detecting at least one of: a voltage at the outputnode that is above the high power supply voltage; or a voltage at theoutput node that is below the low power supply voltage; and a processorconfigured to generate the zero current signal indicative of the zerocrossing of the current though the at least one of the plurality ofmotor windings in accordance with the at least one comparator outputsignal.
 13. The electronic circuit of claim 12, wherein the detectingcomprises: detecting the reverse current by sampling a voltage at theoutput node only at times when the first and second transistors are bothturned off.
 14. The electronic circuit of claim 8, wherein each one ofthe plurality of motor drive signal comprises: a respective plurality ofpulse width modulated signals, each one of the plurality of pulse widthsignals having high states comprising steady state high values proximateto the high power supply voltage and transient high values higher thanthe higher power supply voltage, and each one of the plurality of pulsewidth signals also having low states comprising steady state low valuesproximate to the low power supply voltage and transient low values lowerthan the lower power supply voltage.
 15. A method of driving amulti-phase motor having a plurality of motor windings, the methodcomprising: generating a zero current signal indicative of a zerocrossing of a current though at least one of the plurality of motorwindings; generating a position reference signal indicative of areference position of an angular rotation of the motor; comparing aphase of the zero current signal with a phase of the position referencesignal to generate a phase comparison signal; generating a plurality ofmodulation signals, each having a phase related to a value of the phasecomparison signal; and generating a plurality of motor drive signals tothe plurality of motor windings in accordance with the plurality ofmodulation signals, wherein the generating the zero current signalcomprises: generating the plurality of motor drive signals with arespective plurality of half bridge circuits coupled to the electricmotor, each half bridge circuit comprising: respective first and secondseries coupled transistors; a respective power supply high voltage nodefor receiving a high power supply voltage; a respective power supply lowvoltage node for receiving a low power supply voltage; and a respectiveoutput node at which a respective one of the plurality of motor drivesignals is generated; detecting a reverse current passing through atleast one of the first or the second transistors of at least one of theplurality of half bridge circuits, wherein the detecting comprises atleast one of: detecting a voltage at the output node that is above thehigh power supply voltage; or detecting a voltage at the output nodethat is below the low power supply voltage; and, wherein the methodfurther comprises: generating a zero current signal indicative of a zerocrossing of a current though at least one of the plurality of motorwindings in accordance with the detecting the reverse current.
 16. Themethod of claim 15, wherein the generating the position referencesignals comprises: generating a Hall sensor signal with a Hall sensordisposed proximate to the motor.
 17. The method of claim 15, wherein thegenerating the position reference signals comprises: generating a backEMF signal with a back EMF module coupled to at least one of theplurality of motor windings.
 18. The method of claim 15, wherein thegenerating the position reference signal comprises: stopping a motordrive signal to at least one of the plurality of motor windings during atime window proximate to the motor achieving the reference position; anddetecting the reference position during the time window by a zerocrossing of a back EMF signal.
 19. The method of claim 15, wherein thegenerating the plurality of modulation signals comprises: providing alook up table in which modulation values are stored corresponding to ashape of at least one of the plurality of modulation signals; generatinga first continuous sawtooth ramp signal having a smallest value and alargest value and a plurality of values between the smallest and largestvalues; adding to the first continuous sawtooth ramp signal a valuerelated to the phase comparison signal to generate a second continuoussawtooth ramp signal having the smallest value and the largest value andthe plurality of values between the smallest and largest values, whereinthe smallest and largest values of the second continuous sawtooth rampsignal are shifted in time by an adjustment time from the smallest andlargest values of the first continuous sawtooth ramp signal, wherein theadjustment time is related to the phase comparison signal; using theadjusted continuous sawtooth ramp signal to sequentially look up valuesin the look up table to generate the at least one of the plurality ofmodulation signals; and generating at least one other modulation signalat a predetermined phase relationship to the at least one of theplurality of modulation signals.
 20. The method of claim 15, wherein thedetecting the reverse current comprises: detecting the reverse currentby sampling a voltage at the output node only at times when the firstand second transistors are both turned off.
 21. The method of claim 15,wherein each one of the plurality of motor drive signal comprises: arespective plurality of pulse width modulated signals, each one of theplurality of pulse width signals having high states comprising steadystate high values proximate to the high power supply voltage andtransient high values higher than the higher power supply voltage, andeach one of the plurality of pulse width signals also having low statescomprising steady state low values proximate to the low power supplyvoltage and transient low values lower than the lower power supplyvoltage.
 22. An electronic circuit for driving a multi-phase motorhaving a plurality of motor windings, the electronic circuit comprising:a current measure module configured to generate a zero current signalindicative of a zero crossing of a current though at least one of theplurality of motor windings; a position measure module configured togenerate a position reference signal indicative of a reference positionof an angular rotation of the motor; a modulation signal generationmodule configured to compare a phase of the zero current signal with aphase of the position reference signal to generate a phase comparisonsignal, and configured to generate a plurality of modulation signals,each having a phase related to a value of the phase comparison signal;and a drive circuit configured to generate a plurality of motor drivesignals to the plurality of motor windings in accordance with theplurality of modulation signals; a plurality of half bridge circuitscoupled to the electric motor and configured to generate the pluralityof motor drive signals, each half bridge circuit comprising: respectivefirst and second series coupled transistors; a respective power supplyhigh voltage node for receiving a high power supply voltage; arespective power supply low voltage node for receiving a low powersupply voltage; and a respective output node at which a respective oneof the plurality of motor drive signals is generated, the electroniccircuit further comprising: at least one comparator configured togenerate a respective at least one comparator output signal indicativeof a reverse current passing through at least one of the first or thesecond transistors of at least one of the plurality of half bridgecircuits by detecting at least one of: a voltage at the output node thatis above the high power supply voltage; or a voltage at the output nodethat is below the low power supply voltage; and, the electronic circuitfurther comprising: a processor configured to generate the zero currentsignal indicative of the zero crossing of the current though the atleast one of the plurality of motor windings in accordance with the atleast one comparator output signal.
 23. The electronic circuit of claim22, wherein the position measure module is further configured togenerate the position signal in accordance with a Hall signal generatedby a Hall sensor disposed proximate to the motor.
 24. The electroniccircuit of claim 22, wherein the position measure module is furtherconfigured to generate the position signal in accordance with a back EMFsignal generated in a motor winding.
 25. The electronic circuit of claim22, wherein the generating the position reference signal comprises:stopping a motor drive signal to at least one of the plurality of motorwindings during a time window proximate to the motor achieving thereference position; and detecting the reference position during the timewindow by a zero crossing of a back EMF signal.
 26. The electroniccircuit of claim 22, wherein the modulation signal generation modulecomprises: a look up table in which modulation values are storedcorresponding to a shape of at least one of the plurality of modulationsignals; a sawtooth generator configured to generate a first continuoussawtooth ramp signal having a smallest value and a largest value and aplurality of values between the smallest and largest values; atiming/phase error detector coupled to receive a signal representativeof the zero current signal, coupled to receive a signal representativeof the position reference signal, and configured to generate a signalrepresentative of the phase comparison signal; and a summation moduleconfigured to add to the first continuous sawtooth ramp signal a valuerelated to the phase comparison signal to generate a second continuoussawtooth ramp signal having the smallest value and the largest value andthe plurality of values between the smallest and largest values, whereinthe smallest and largest values of the second continuous sawtooth rampsignal are shifted in time by an adjustment time from the smallest andlargest values of the first continuous sawtooth ramp signal, wherein theadjustment time is related to the phase comparison signal, wherein theadjusted continuous sawtooth ramp signal is used to sequentially look upvalues in the look up table to generate the at least one of theplurality of modulation signals and also used to generate at least oneother modulation signal at a predetermined phase relationship to the atleast one of the plurality of modulation signals.
 27. The electroniccircuit of claim 22, wherein the detecting comprises: detecting thereverse current by sampling a voltage at the output node only at timeswhen the first and second transistors are both turned off.
 28. Theelectronic circuit of claim 22, wherein each one of the plurality ofmotor drive signal comprises: a respective plurality of pulse widthmodulated signals, each one of the plurality of pulse width signalshaving high states comprising steady state high values proximate to thehigh power supply voltage and transient high values higher than thehigher power supply voltage, and each one of the plurality of pulsewidth signals also having low states comprising steady state low valuesproximate to the low power supply voltage and transient low values lowerthan the lower power supply voltage.